1. Field of the Invention
The present invention relates to a apparatus for detecting a flow rate, and more particularly to, for example, a flow sensor for use in an internal combustion engine or a flow sensor for use in a fuel battery system.
2. Description of the Related Art
Hitherto, as air flow sensors disposed in intake air passages of internal combustion engines of automobiles, etc. for measuring the amounts of intake air, thermal ones have been primarily employed because of a capability of directly detecting air mass flow rates. Recently, air flow sensors manufactured by semiconductor micromachining techniques, in particular, have received attention because they have a high-speed response and are able to detect a back flow by utilizing the high-speed response.
One example of the conventional technique regarding such a thermal type air flow sensor using a semiconductor substrate is disclosed in, e.g., Japanese Publication of Examined Patent Application No. 6-63801. In the related art, an electric current is supplied to a heater resistance arranged between an upstream-side temperature sensor and a downstream-side temperature sensor for generating heat, and a flow rate signal is obtained depending on a difference in output signal between the upstream-side temperature sensor and the downstream-side temperature sensor.